Magnetic Sensors are used in navigation systems for sensing the direction of the horizontal component of the earth's magnetic field to implement navigating a vehicle along a desired course. Although in principle, at least, an ordinary magnetic compass can be used for this purpose, magnetic sensors have the advantage of delivering the required information electrically, thus enabling its use in a great variety of ways.
A fluxgate sensor is a common type of magnetic sensor that is well known in the art. While the present invention will be described with reference to a fluxgate sensor, it is to be understood that any magnetic sensor capable of delivering a signal to find the heading of a carrier vehicle can be used with the invention.
The accuracy of magnetic sensors is impaired due to the self-magnetism of the carrier vehicles. In order to overcome this, the sensor is generally located on the most magnetically isolated part of the vehicle. In aircraft, for example, this is the wingtips or other structure far remote from the ferrous masses of the engines. In seacraft, the sensor is normally mounted high on a mast, away from the magnetic masses of the hull and engines.
Nevertheless, errors in the sensor's heading readings due to permanent and permeable magnetic effects in the carrier vehicle do occur, and these are usually compensated for by a variety of means well known in the art. Principally, local magnetic fields are generated that oppose those from the carrier vehicle through the use of magnets or coils through which currents are passed. Preferably, the compensating currents are passed into the windings of the sensor itself.
A major source of sensor error is the vertical component of the earth's field. In order to minimize this error, the sensors are pendulously-mounted, or gyro mounted, so that the sensor's sensitive or input axis is maintained in the true horizontal plane, whereby the sensor responds to the direction of the horizontal magnetic component of the earth's field and is perpendicular to the earth's vertical magnetic component, so that the latter will not induce errors even when the carrer vehicle experiences pitch and/or roll motion.
In the case of applications to air or seacraft, the pendulously-mounted sensor, together with its error-compensation means, has been effective and accurate, but this is due in large measure to the megnetically favorable locations available for mounting the sensor on these vehicles as aforenoted.
More recently, magnetic navigation systems employing magnetic sensors have been applied to land vehicles such as steel trucks, military battle-tanks, and the like. In these applications, it has been found that the vehicle's magnetic disturbances are much more severe, primarily because operational considerations demand that the sensor be mounted close to, or actually upon, the vehicle's surface. Analyses and measurements made for such vehicles indicate that compensation techniques currently available are relatively ineffective especially when the vehicle pitches and/or rolls, or such compensation techniques are required to be extremely complex and costly. Hence, the accuracy of an electrical compass system on such vehicles with current state-of-the-art compensation techniques is often not acceptable.
The compensated magnetic sensor to be described herein is addressed particularly, but not exclusively, to applicatons in highly magnetized land vehicles such as the aforenoted steel trucks or military battle-tanks, and is more simple, less costly and more accurate, particularly when the vehicle pitches and/or rolls, or remains stationary on steep inclines, than like compensated magnetic sensors now known in the art.